The interface to be satisfied by implementations maintaining collections
of RDF triples. The core interface is small (add, delete, find, contains) and
is augmented by additional classes to handle more complicated matters
such as event management.

remove

close

isEmpty

boolean isEmpty()

Answer true iff this graph is empty. "Empty" means "has as few triples as it
can manage", because an inference graph may have irremovable axioms
and their consequences.

size

int size()

For a concrete graph this returns the number of triples in the graph. For graphs which
might infer additional triples it results an estimated lower bound of the number of triples.
For example, an inference graph might return the number of triples in the raw data graph.